Custom themes / colorschemes

Hey @skwp, I made the PR again. This time I didn't include my personal theme...
If anyone is interested they can check my fork :)

**So, if you wanna change the look of your vim to your own color scheme:**

  * Define the name of the colorscheme in `yadr-appearance.vim`
  * If you wanna do any tuning on top of that colorscheme, place them on `colors-settings/<your_colorscheme_name>.vim` and it will automatically be applied right after applying the colorscheme itself.
  * If you wanna do any tuning on top of the powerline theme, place them on `colors-settings/<your_colorscheme_name>-powerline.vim`

**Notes**

  * I had to move these files out of the plugins/settings/ folder because all files under /plugins are automatically source by vim when you open it and we don't want that if we  want to provide this ability of changing colorscheme (solarized might define a property that my theme doesn't and vice&versa so this would mess things up).
  * If you don't provide a `<your_theme>-powerline.vim`, I've set it to fallback to `solarized-powerline.vim` otherwise it would look just too ugly :D